Press working method

ABSTRACT

The invention offers a pressed product (F) produced by press-forming a metal plate ( 1 ). The pressed product (F) has a peripheral surface that has a corner portion ( 12 ) connecting two surfaces in the peripheral surface. The corner portion ( 12 ) has an outside corner radius R that is equal to or smaller than the thickness “t” of the metal plate ( 1 ). That is, the pressed product (F) has the sharp corner portion ( 12 ).

TECHNICAL FIELD

The present invention relates to a pressed product to be used, forexample, for the housing of a portable electronic device, particularlyto a pressed product having a sharp corner portion.

BACKGROUND ART

Metal such as aluminum or its alloy is used as the material for thehousing of a portable electronic device or the like, such as a cellularphone and a note-book-type personal computer. Generally, in comparisonwith resin, metal has a higher strength and is more resistant to animpact.

As the foregoing material for the housing, magnesium alloys have beenused which are formed by adding various elements to magnesium. Althougha magnesium alloy has excellent specific strength and specific rigidity,it has poor plastic workability at ordinary temperatures because it hasa hexagonal crystalline structure (hexagonal close-packed structure).Consequently, the housing and the like are mainly formed by using a castproduct produced by a die-casting process or a thixomold process. Inrecent years, engineers have been studying the performing of pressworking on a magnesium alloy (Patent Literatures 1 and 2).

SUMMARY OF INVENTION Technical Problem

A typical shape of the housing is a box type provided with a rectangulartop plate and four side walls formed from the edges of the top plate.For the box-type shaped body, the market desires to obtain a housinghaving sharpness both at the corner portion connecting the top plate andthe side wall and at the corner portion connecting two side walls. It islikely that a housing having sharp corner portions can be formed whenthe injection molding of resin or casting is used. Nevertheless, aresinous product and a cast product generally have a lower strength thanthat of a pressed product of metal.

When the box-type shaped body falls and collides with the ground or thelike, the impact at the time of the collision is applied to theabove-described corner portions in many cases. As a result, in aresinous product and a cast product both having low strength, the cornerportion is deformed (broken) or otherwise damaged, so that it isdifficult for the corner portion to maintain the sharp condition.

Solution to Problem

The present invention is made in view of the foregoing circumstances andoffers a shaped body having a sharp corner portion and having highstrength. More specifically, the present invention offers a pressedproduct produced by press-forming a metal plate. The pressed product hasa peripheral surface that has a corner portion connecting two surfacesin the peripheral surface. The corner portion has an outside cornerradius R satisfying the condition that R is equal to or smaller than(⅔)×t, where “t” is the thickness of the metal plate. The pressedproduct is formed by using a material including 8.3 mass % or more and9.5 mass % or less Al, 0.5 mass % or more and 1.5 mass % or less Zn, andthe remainder that includes Mg and impurities. In the pressed product:

(a) the outside corner radius R is 0.2 mm or more and 0.4 mm or less,

(b) the thickness “t” is 0.4 mm or more and 0.8 mm or less, and

(c) the corner portion has a hardness of 90 Hv or more.”

According to the above structure, by performing the press forming on ametal plate, the hardness of the corner portion is increased by the workhardening owing to the plastic working. Consequently, even when animpact is applied to the corner portion and the like, deformation isless likely to occur, so that the sharp corner portion can be maintainedfor a long time. Moreover, because the pressed product of the presentinvention is formed by the press forming of a metal plate, in additionto the strength of the material itself, the strength can be increased bythe plastic working, so that the entire pressed product has highstrength. Furthermore, because the pressed product of the presentinvention has the foregoing sharp corner portion, it can give animpression of stylishness with a refined design. As a result, it isexpected that the pressed product has excellent appearance as acommodity and therefore has an enhanced commercial value.

The above-described pressed product of the present invention having asharp corner portion can be produced, for example, by performing thebelow-described multistage press working on a blank plate made of metal.More specifically, the production method is to produce a pressed producthaving a corner portion by performing press working on a metal plate andhas the steps described below.

A step of preparing a blank plate: this step prepares a blank plate madeof metal.

A first pressing step: this step produces a pressed material that has atleast one corner portion connecting two surfaces in the peripheralsurface under the condition that the blank plate is heated at atemperature of 200° C. or more and 300° C. or less. In particular, thefirst press working is performed so that at least one corner portion canhave an inside corner radius “r” that is practically 0 mm by using apunch having a shoulder portion with a corner radius Rp that ispractically 0 mm.

A second pressing step: this step produces a pressed product that has atleast one corner portion having an outside corner radius R that is equalto or smaller than the thickness “t” of the metal plate by performingthe second press working under the condition that the foregoing pressedmaterial is heated at a temperature of 200° C. or more and 300° C. orless. In particular, the second press working is performed so that theabove-described corner portion, which has an inside corner radius “r”that is practically 0 mm, can have an outside corner radius R that isequal to or smaller than the foregoing thickness “t” by using astep-shaped punch for pressing both the end face of the pressed materialand the corner portion, which has been formed on the inside surface inthe first pressing step and which has “r” that is practically 0 mm.

A sharp corner portion having an outside corner radius R that is equalto or smaller than the thickness “t” of the metal plate can becomeeasily formed when the blank plate to be pressed has the thinnestpossible thickness. Nevertheless, when the blank plate itself isexcessively thin, the strength of the pressed product is decreased, sothat it cannot satisfy the strength and rigidity required for thehousing of a portable electronic device. On the other hand, to increasethrough the work hardening the hardness of the corner portion of apressed product, the corner portion being likely to suffer from animpact at the time of the falling, it can be conceived to form thecorner portion at a high working ratio. However, when the bending ordeep drawing is performed at a high working ratio, the cornerportion-formed place in the blank plate is partially elongated reducingits thickness. This thickness decrease leads to the reduction instrength.

In consideration of the foregoing phenomenon, the above-describedproduction method carries out the press forming at a high working ratioby dividing the process into multiple stages as described above, not bya process of one stage. Consequently, the corner portion is preventedfrom becoming extremely thin, so that the reduction in strengthresulting from the thickness reduction can be suppressed. As a result,the above-described production method can not only produce a pressedproduct having a sharp corner portion but also maintain the sharp cornerportion for a long period.

In addition, the above-described production method performs the pressworking under a heated condition. Consequently, even a metal that haspoor plastic workability and therefore develops springback, cracking, orthe like in the cold working, such as magnesium alloy having anelongation of 20% or so at the most at ordinary temperatures, canincrease the elongation of the object to be worked (the blank plate andpressed material) to 100% or more at the time of the pressing.Furthermore, because the object to be worked has a sufficientelongation, a pressed product that has a corner portion having anextremely small outside corner radius R can be produced with highprecision.

Advantageous Effects of Invention

The pressed product of the present invention has a sharp corner portionand high strength.

According to the above-described production method, a corner portionformed on the peripheral surface, that is, the corner portion formingthe appearance, is sharp. In addition, a corner portion formed on theinside surface is also sharp. Consequently, the pressed product has anample internal space. As a result, when the pressed product obtained bythe foregoing production method is used for a housing, variouscomponents can be adequately housed in the housing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view for explaining the procedureof the press working in the process of producing a pressed producthaving a sharp corner. The (A) portion of FIG. 1 shows a state in whicha blank plate is placed in a die. The (B) portion of FIG. 1 shows astate in which a pressed material is formed by the first press working.The (C) portion of FIG. 1 shows a state in which a punch is performingpressing in the second press working. The (C′) portion of FIG. 1 is apartially enlarged view of the corner portion shown in the (C) portion.The (D) portion of FIG. 1 shows a state in which a pressed producthaving a sharp corner portion is formed by the second press working. The(E) portion of FIG. 1 shows an obtained pressed product.

DESCRIPTION OF EMBODIMENTS

An explanation is given below to the embodiments of the presentinvention. In the explanation of the drawings, the same component bearsthe same sign to eliminate duplicated explanations. The ratio of thedimensions in the drawing does not necessarily coincide with that of theexplanation.

Pressed Product Composition

The pressed product of the present invention may be formed by usingvarious metals having excellent press formability at a temperature rangeof 200° C. or more and 300° C. or less. In particular, it is desirableto use magnesium or its alloy as the material for forming, for example,the housing of a portable electronic device required to have lightweight, because magnesium and its alloy are lightweight, have highstrength, and have excellent impact resistance.

It is possible to use magnesium alloys having various compositionsformed by adding various elements to Mg (the remainder: Mg andimpurities). The types of magnesium alloy include Mg—Al-based alloy,Mg—Zn-based alloy, Mg-RE (rare-earth element)-based alloy, and Y-addedalloy. In particular, Mg—Al-based alloy, which contains Al, has highcorrosion resistance. The types of Mg—Al-based alloy include, asspecified in the Standards of American Society for Testing and Materials(ASTM), AZ-family alloy (Mg—Al—Zn-based alloy, Zn: 0.2 to 1.5 mass %),AM-family alloy (Mg—Al—Mn-based alloy, Mn: 0.15 to 0.5 mass %),AS-family alloy (Mg—Al—Si-based alloy, Si: 0.6 to 1.4 mass %), andMg—Al-RE (rare-earth element)-based alloy. It is desirable that the Alcontent be 1.0 to at most 11 mass %. In particular, an Mg—Al-based alloycontaining 8.3 to 9.5 mass % Al and 0.5 to 1.5 mass % Zn, represented byAZ91 alloy, has excellent corrosion resistance and mechanical propertiessuch as strength and resistance to plastic deformation in comparisonwith other Mg—Al-based alloys such as AZ31 alloy. The types ofMg—Zn-based alloy, containing Zn, include, as specified in the ASTMStandards, ZK-family alloy (Mg—Zn—Zr-based alloy, Zr: 3.5 to 6.2 mass %,Zn: 0.45 mass % or more).

Shape

The pressed product of the present invention is formed by performingpress working, such as bending and deep drawing, on a metal plate. Inthe pressed product, the peripheral surface has at least one cornerportion that connects two surfaces in the peripheral surface. A typicalshape of the pressed product has a top-plate portion (an undersurfaceportion) and a side-wall portion formed from the edge of the top-plateportion. More specifically, the types of the shape include a box-shapedbody having a top-plate portion with the shape of a rectangular plateand only one pair of opposed side-wall portions, a box-shaped bodyhaving two pairs of opposed side-wall portions, and a lidded hollowcylindrical body having a top-plate portion with the shape of a circularplate and a hollow cylindrical side-wall portion. The top-plate portionand the side-wall portion each have an outside surface and an insidesurface that are practically parallel to each other. In the pressedproduct of the present invention, the corner portion connecting twooutside surfaces is sharp.

The above-described top-plate portion and side-wall portion are eachtypically formed with a flat surface; their shape and size are notparticularly limited. They may have a unitarily formed or bonded boss orthe like. They may have a through hole penetrating from the outsidesurface to the inside surface or a groove recessed in the thicknessdirection. They may have a stepped shape. They may have a portion havinga locally different thickness, the portion being formed by plasticworking or cutting. The portion other than the corner portion has anearly uniform thickness, except the above-described portions having aboss, a recessed portion, and a different thickness. Hence, thethickness “t” of the metal plate forming the pressed product is definedas the average thickness of the entire portion excluding the foregoingboss and the like and the corner portion. More specifically, five ormore measuring spots are chosen from the portion other than theabove-described portions such as the boss to obtain the average value.When the top-plate portion is formed of a flat surface, the foregoingthickness “t” may be the average thickness of the entire top-plateportion. The thickness “t” mostly depends on the thickness of the blankplate. In the case where the plate is not subjected to the pressworking, the thickness “t” is practically the same as that of the blankplate.

When the above-described metal plate has a thickness “t” of 0.4 mm ormore and 2.0 mm or less, a pressed product having excellent strength isobtained. When the thickness is less than 0.4 mm, because the blankplate is thin, it is easy to form a pressed product having a smalleroutside corner radius R. As a result, a pressed product having veryexcellent appearance, such as stylishness, is obtained. When the metalplate has a thickness “t” of 2.0 mm or less, particularly 1.5 mm orless, yet particularly 0.6 mm or less, it is expected that the pressedproduct of the present invention can be suitably used, for example, forthe housing of a portable electronic device.

The most noticeable feature of the pressed product of the presentinvention is that in the peripheral surface, at least one corner portionconnecting two surfaces in the peripheral surface has an outside cornerradius R that is equal to or smaller than “t.” The conventional pressedproduct made of a metal plate has had an outside corner radius largerthan the thickness of the metal plate; conventionally, no pressedproduct has satisfied the condition that R is equal to or smaller than“t.” In contrast, the pressed product of the present invention has atleast one corner portion satisfying the condition that R is equal to orsmaller than “t.” When all corner portions existing in the pressedproduct of the present invention satisfy the condition that R is equalto or smaller than “t,” it is expected that the stylishness can befurther enhanced.

In particular, when the outside corner radius R satisfies the conditionthat R is equal to or smaller than (⅔)×t, the corner portion can easilyhave a hardness higher than that of the portion other than the cornerportion, such as the top-plate portion, so that the corner portion canhave improved impact resistance. When the condition that R is equal toor smaller than (½)×t is satisfied, the impact resistance of the cornerportion and the stylishness can be further improved. More specifically,it is desirable that the outside corner radius R be 0.1 to 0.3 mm. Whenthe outside corner radius R is 0.1 mm or more, the possibility of beingcut or damaged by a sharp corner portion can be reduced. When R is 0.3mm or less, the corner portion can have excellent impact resistancewhile obtaining good appearance. The decreasing of the outside cornerradius R can be achieved, for example, by increasing the pressingpressure in the second pressing step.

In the pressed product obtained by the above-described productionmethod, at the corner portion that satisfies the condition that R isequal to or smaller than “t,” the inside corner radius “r” becomespractically 0 mm. In other words, in the inside surface, the twosurfaces positioned at the inside of the above-described corner portionare practically perpendicular to each other. Such a pressed product hasan ampler internal space than the pressed product having an insidecorner radius “r” that is larger than zero and therefore can be suitablyused for a housing to be used to house various components.

Production Method Preparation of the Blank Plate

When magnesium alloy is used to form the blank plate, it is desirable touse a rolled sheet produced by rolling a plurality of times a cast sheetproduced by a continuous casting process such as the twin-roll process,particularly the casting process stated in WO/2006/003899. Because thecontinuous casting process enables the rapid solidification, thecreation of oxides and segregation can be decreased, so that a castsheet having excellent rolling workability can be obtained. When a castsheet is subjected to heat treatment such as the solution treatment(heating temperature: 380° C. to 420° C., heating time: 60 to 600minutes) or the aging treatment, the composition can be homogenized. Inparticular, in the case of a magnesium alloy having a high Al content,it is desirable to perform the solution treatment for a long time. Thesize of the cast sheet is not particularly limited. Nevertheless, if thesheet is excessively thick, segregation tends to be created. Hence, itis desirable that the thickness be 10 mm or less, particularly 5 mm orless.

When a plurality of rolling operations are performed on theabove-described cast sheet, the desired sheet thickness can be obtainedand the average crystal grain size can be decreased. In addition, for amagnesium alloy having a high Al content, such as AZ91 alloy, defectssuch as coarse impurities in crystal and coarse precipitated impuritiescan be eliminated, so that the press workability can be increased. Therolling operation may be performed by combining with a well-knowncondition, for example, in the case of magnesium alloy, the controlledrolling disclosed in Patent Literature 2 or the like. When during thecourse of the rolling operation, an intermediate heat treatment (heatingtemperature: 250° C. to 350° C., heating time: 20 to 60 minutes) isconducted to remove or decrease the strain, residual stress, aggregatedtexture, and so on all introduced into the object to be worked throughthe working before the intermediate heat treatment, the subsequentrolling operation can be performed more smoothly by preventingaccidental cracking, strain, and deformation. The obtained rolled sheetmay undergo a heat treatment at 300° C. or more to remove the workstrain resulting from the rolling operation and to achieve completerecrystallization. Alternatively, the obtained rolled sheet may acquirestrain through the use of a roller leveler or the like under a heatedcondition to be recrystallized during the press working.

Press Working

It is desirable that the press working in multiple stages be performedin a temperature range of 200° C. to 300° C. in every stage in order toincrease the plastic workability of the object to be worked (the blankplate and pressed material).

In the first press working, the use of a punch having a shoulder portionwith a corner radius Rp nearly equal to zero (desirably, the cornerradius Rp is 0.3 mm or less) forms a pressed material having a sharpcorner portion (practically right-angled) at the inside, that is, apressed material having an inside surface in which a corner portion isformed by two surfaces perpendicular to each other. In the second pressworking, the end face of the pressed material and the foregoing insidecorner portion are pressed with a step-shaped punch. Thus, constitutingmaterials of the blank plate are forcefully gathered at the cornerportion of the die, and the forcefully gathered constituting materialsare deformed in such a way that a sharp corner portion is formed at theoutside of the pressed material. At the time of the press working, asuitable die is used, such as a movable die or a recessed die.

After the press forming, heat treatment may be conducted in order toremove the strain and residual stress introduced by the press workingand to improve the mechanical property. An example of the heat treatmentcondition is as follows: heating temperature: 100° C. to 450° C.,heating time: 5 minutes to 40 hours or so. When the pressed productobtained by the press working is provided with a covering layer aimingat corrosion proofing, protection, and ornamentation, the corrosionresistance, commercial value, and so on can be further increased.

Test Example 1

A plurality of pressed products each made of magnesium alloy and havingcorner portions were produced to examine the outside corner radius R ofthe corner portions and the hardness of the pressed products.

A plurality of cast sheets (thickness: 4 mm) were prepared that weremade of magnesium alloy having a composition equivalent to that of AZ91alloy (Mg, 9.0 mass % Al, and 1.0 mass % Zn) and that were produced bythe twin-roll continuous casting process. The obtained cast sheets weresubjected to a plurality of rolling operations until the thickness wasreduced to 0.6 mm under the following rolling conditions: rollertemperature: 150° C. to 250° C., sheet temperature: 200° C. to 400° C.,and rolling reduction per pass: 10% to 50%. The obtained sheet materialswere subjected to blanking to prepare blank plates for the pressforming.

The prepared blank plates underwent two stages of press working. FIG. 1is a schematic cross-sectional view for explaining the procedure of thepress working. FIG. 1 shows the blank plate by emphasizing it.

The first press working produces a pressed material P (a box-shaped bodyhaving outside dimensions of 45×95×6 mm) having a flat top-plate portion10 as shown in the (B) and (C) portions of FIG. 1 and two pairs of flatside-wall portions 11 formed from the top-plate portion 10. Morespecifically, as shown in the (A) portion of FIG. 1, a blank plate Bhaving a thickness “t” of 0.6 mm is placed on a plate 51 and a die plate52, and on the blank plate B, a punch 53 and a holding plate 54 areplaced. Then, under the condition that the blank plate B is sandwichedbetween the plate 51 and the punch 53, the punch 53 is moved downward inFIG. 1 to form the pressed material. The punch 53 has a shoulder portionwhose corner radius Rp is practically 0 mm, and the two surfaces formingthe shoulder portion are perpendicular to each other. In the pressedmaterial P produced by this press working, the corner portion 12, whichconnects the outside surface 10 o of the top-plate portion 10 and theoutside surface 11 o of the side-wall portion 11, has an outside cornerradius R₀ that is larger than the thickness t₀ of the top-plate portion10. In addition, the inside surface 10 i of the top-plate portion 10 andthe inside surface 11 i of the side-wall portion 11 are perpendicular toeach other. In other words, the inside corner radius r₀ of the cornerportion 12 is practically 0 mm. In the above description, the plate 51,the die plate 52, the punch 53, the holding plate 54, and thebelow-described stepped punch 55 and die 56 all can be heated with aheating means, which is unshown. In the first press working, the heatingtemperature was 200° C. or more.

The second press working uses, for example, as shown in the (C) and (D)portions of FIG. 1, a protrusion-type stepped punch 55 and a die 56having a recessed portion to press an end face 11 e of the side-wallportion 11 of the pressed material P. This pressing operation produces apressed product F that has a sharp corner portion whose outside cornerradius R is equal to or smaller than the thickness “t” of a metal plate1. The stepped punch 55 has an end-portion-pressing face 55 p and ashoulder portion 55 s that, at the inside surface of the pressedmaterial P, is brought into contact with an inside corner portion 12 i,which is nearly right-angled, to press the inside corner portion 12 i.In the recessed portion of the die 56, a bottom face 56 b and a sideface 56 s are perpendicular to each other, so that the corner portion isright-angled.

As shown in the (C) and (C′) portions of FIG. 1, under the conditionthat the pressed material P is placed in the die 56, which has arecessed portion fitting to the outer contour of the box-shaped pressedmaterial P, the inside of the pressed material P is pressed with thestepped punch 55. At this moment, first, the end-portion-pressing face55 p of the stepped punch 55 presses the end face 11 e of the side-wallportion 11. Then, when the stepped punch 55 continues the downwardpressing, a main pressing face 55 m of the stepped punch 55 is broughtinto contact with the inside surface 10 i of the top-plate portion 10 topress it. When the inside corner portion 12 i of the pressed material Pis pressed with the shoulder portion 55 s of the stepped punch 55,constituting materials both in a part of the side-wall portion 11 and ina part of the top-plate portion 10 are forcefully gathered at the cornerportion of the recessed portion of the die 56. As a result, a sharpcorner portion is formed in the pressed material P in accordance withthe corner portion of the recessed portion. In the above description, inorder to facilitate uniform pressing of the end face 11 e of theside-wall portion 11, part of the side-wall portion of the pressedmaterial obtained by the first press working is worked by the sidecutting, so that the pressed material has a side-wall portion with auniform height of 5.5 mm. Under this condition, the second press workingwas carried out. In the second press working, the magnitude of theoutside corner radius R was varied by varying the pressure at the timeof the pressing.

Through the above-described process, as shown in the (E) portion of FIG.1, a pressed product F can be obtained in which a corner portion 22 thatconnects an outside surface 20 o of a top-plate portion 20 and anoutside surface 21 o of a side-wall portion 21 has an outside cornerradius R that is equal to or smaller than the thickness “t” of thetop-plate portion 20. When the thickness of the top-plate portion 20 andthe side-wall portion 21 was measured using a pointed micrometer, thethickness was 0.6 mm, which is nearly equal to the thickness of theblank plate B.

The obtained pressed product was subjected to the measurements of theoutside corner radius R, the hardness of the corner portion, and thehardness of the top-plate portion. The results are shown in Table I.

The outside corner radius R (mm) was measured by the following method.First, a pressed product was cut in a direction perpendicular to theridge line between the outside surface of the top-plate portion and theoutside surface of the side-wall portion. The cut surface wasbuff-polished (using diamond abrasive grain No. 200) and then observedunder an optical microscope (400 power). The observed image was used tomeasure the radius. Similarly, the inside corner radius “r” (mm) of thecorner portion was measured. The result showed that the radius waspractically 0 mm and the inside surface of the top-plate portion and theinside surface of the side-wall portion were practically perpendicularto each other.

The hardness Hv of the corner portion was measured by the followingmethod. First, a pressed product was cut in a direction perpendicular tothe ridge line between the outside surface of the top-plate portion andthe outside surface of the side-wall portion. The obtained cut piece wasused to produce an embedded specimen. The cut surface wasmirror-polished. Three measuring spots were chosen from the centerportion of the plate in the thickness direction in the cut surface ofthe pressed product. The hardness of the individual measuring spots wasmeasured using a micro-Vickers hardness tester. The average value of thethree measurements is shown in Table I.

The hardness Hv of the top-plate portion was measured by the followingmethod. First, a part of the top-plate portion was cut from the pressedproduct. The obtained cut piece was used to produce an embeddedspecimen. The cut surface was mirror-polished. Three measuring spotswere chosen from the center portion of the plate in the thicknessdirection in the cut surface of the top-plate portion. The hardness ofthe individual measuring spots was measured using a micro-Vickershardness tester. The average value of the three measurements is shown inTable I. Alternatively, the hardness Hv may also be measured byproducing an embedded specimen including both the corner portion andtop-plate portion.

TABLE I Thickness of metal plate (mm) 0.6 Sample No. 1-1 1-2 1-3 1-4 1-5Outside corner 0.6 0.5 0.4 0.3 0.2 radius R (mm) Relation between R is Ris R is R is R is R and t equal larger equal equal smaller to t than toto than (⅔)t (⅔)t (½)t (½)t Hardness of corner 85 86 90 95 97 portion(Hv) Hardness of top-plate 85 86 85 85 86 portion (Hv)

As can be seen from Table I, the performing of the multistage hot pressworking described above can produce with high precision pressed productswhose outside corner radius R satisfies the condition that R is equal toor smaller than the thickness “t.” In particular, when theabove-described press working is performed on a metal plate, the cornerportion and the top-plate portion have a comparable hardness.Consequently, because these pressed products have corner portions withhigh strength, it is expected that they are less likely to be deformedwhen they undergo an impact such as one due to the falling.

Table I also shows that when the outside corner radius R satisfies thecondition that R is equal to or smaller than (⅔)×t, the hardness of thecorner portion is increased. The reason for this is attributable to thework hardening resulting from the multistage press working. Furthermore,when the outside corner radius R satisfies the condition that R is equalto or smaller than (½)×t, the hardness of the corner portion issignificantly increased. It can be expected that these pressed productshave corner portions that are excellent in impact resistance and thatcan accordingly maintain the sharp condition for a long time.

In Test example 1 described above, an explanation is given to the casewhere the corner portion connecting the top-plate portion and theside-wall portion has an outside corner radius R satisfying thecondition that R is equal to or smaller than “t” in a box-type shapedbody having two pairs of side-wall portions. Similarly, a pressedproduct can also be produced in which a corner portion connecting theside-wall portions has an outside corner radius R satisfying thecondition that R is equal to or smaller than “t.” A pressed producthaving only one pair of side-wall portions can also be produced.Furthermore, in Test example 1 described above, an explanation is givenby referring to the unitarily formed stepped punch. Nevertheless, astepped punch formed by combining divided pieces may also be used. Forexample, a stepped punch may also be used that is provided with adivided piece for pressing mainly the top-plate portion and anotherdivided piece for pressing the end face of the side-wall portion andpart of the top-plate portion.

Test Example 2

Blank plates having various thicknesses were prepared to produce pressedproducts made of magnesium alloy. The produced pressed products weresubjected to examination of strength and appearance.

Cast sheets (thickness: 4 mm) of the same type as prepared in Testexample 1 were prepared. The prepared cast sheets had a compositionequivalent to that of AZ91 alloy and were subjected to a rollingoperation by varying the number of times of rolling to produce rolledsheets having various thicknesses (thicknesses: 0.3 to 0.8 mm). As withTest example 1, the obtained rolled sheets were subjected to blanking toprepare blank plates. As with Test example 1, the individual blankplates underwent two stages of hot press working (the heatingtemperature at the time of pressing was selected as appropriate from therange of 200° C. to 250° C.). Thus, pressed products were produced eachof which had a flat top-plate portion and two pairs of flat side-wallportions formed from the top-plate portion.

The obtained individual pressed products were subjected to themeasurement of the outside corner radius R (mm) of the corner portion bythe same method as used in Test example 1. The results are shown inTable II.

In addition, the strength of the pressed product was measured asdescribed below. The pressed product was placed such that the top-plateportion of the pressed product pointed upward on the side-wall portionused as a supporting member. Under this condition, a cemented-carbideball having a diameter of 38 mm is pressed into the center of thetop-plate portion at a load of 1 kgf (9.8 N) to deform the pressedproduct permanently. The amount of deformation (the dimensionaldifference between the most protruding portion and the most recessedportion in the peripheral surface of the top-plate portion) is measuredwith a contact profilometer. The amount of deformation is evaluated asthe strength of the pressed product. When the amount of deformation is 1mm or more, the pressed product is considered to have an insufficientstrength and evaluated as “poor.” When the amount of deformation is lessthan 1 mm, the pressed product is considered to have a sufficientstrength and evaluated as “good.” When the amount of deformation is lessthan 0.3 mm, the pressed product is considered to have an excellentstrength and evaluated as “excellent.” The results are shown in TableII.

The appearance is evaluated through a panel test conducted by 10panelist chosen randomly. When five or less panelists judged that thepressed product has a sharp and distinctive outside corner portion,stylishness, and excellent designability, the pressed product isevaluated as “poor.” When six to eight panelists judged as describedabove, the pressed product is evaluated as “good.” When nine or morepanelists judged as described above, the pressed product is evaluated as“excellent.” The results are shown in Table II.

TABLE II Thickness of top-plate Outside Relation Amount of EvaluationEvaluation Sample portion corner radius between deformation of of TotalNo. t (mm) R (mm) R and t (mm) strength appearance evaluation 2-1 0.30.2 R is equal to (⅔)t 1.30 Poor Excellent Poor 2-2 0.4 0.2 R is equalto (½)t 0.55 Good Excellent Good 2-3 0.5 0.2 R is equal to or smaller0.35 Good Excellent Good than (½)t 2-4 0.6 0.2 R is equal to or smaller0.25 Excellent Excellent Excellent than (½)t 2-5 0.6 0.3 R is equal to(½)t 0.25 Excellent Excellent Excellent 2-6 0.6 0.4 R is equal to (⅔)t0.25 Excellent Good Good 2-7 0.6 0.5 R is larger than (⅔)t 0.25Excellent Poor Poor 2-8 0.8 0.2 R is equal to or smaller 0.18 ExcellentExcellent Excellent than (½)t Note: Between the evaluation of strengthand the evaluation of appearance, whichever is worse in the evaluationis used as the total evaluation.

As can be seen from Table II, as the thickness “t” of the top-plateportion increases, the strength is increased. In addition, when theoutside corner radius R is equal to or smaller than (½)×t, the strengthis high and the appearance is excellent.

The above-described embodiments may be changed as appropriate withoutdeviating from the gist of the present invention and not limited to theabove-described constitutions. For example, the material of the metalplate may be changed from magnesium alloy to aluminum, its alloy, andother various metals.

INDUSTRIAL APPLICABILITY

The pressed product of the present invention can be suitably used forvarious electronic devices, particularly for the housing of a portableelectronic device or the like.

REFERENCE SIGNS LIST

-   1: metal plate-   10: top-plate portion; 10 o: outside surface of the top-plate    portion; 10 i: inside surface of the top-plate portion-   11: side-wall portion; 11 o: outside surface of the side-wall    portion; 11 i: inside surface of the side-wall portion; 11 e: end    face of the side-wall portion-   12: corner portion; 12 i: inside corner portion-   20: top-plate portion; 20 o: outside surface of the top-plate    portion; 21: side-wall portion;-   21 o: outside surface of the side-wall portion; 22: corner portion-   51: plate; 52: die plate; 53: punch; 54: holding plate; 55: stepped    punch; 55 m: main pressing face; 55 p: end-portion-pressing face; 55    s: shoulder portion; 56: die; 56 b: bottom face; 56 s: side face-   B: blank plate; P: pressed material; F: pressed product

CITATION LIST Patent Literature

-   PTL 1: the published Japanese patent application Tokukai 2002-239644-   PTL 2: the published Japanese patent application Tokukai 2007-098470

1-9. (canceled)
 10. A method for press working a metal plate, the methodcomprising: a first step of forming a pressed material having a sidewall portion and an inside corner portion by pressing a first punch onthe metal plate, the first punch including a shoulder portion having acorner radius Rp of 0.3 mm or less; and a second step of forming apressed product by pressing with a second punch, the inside cornerportion and an end face of the side wall portion of the pressedmaterial.
 11. The method according to claim 10, wherein the second stepof forming the pressed product includes: forming a thickness of atop-plate portion of the pressed product and a thickness of theside-wall portion of the pressed product substantially uniform.
 12. Themethod according to claim 10, wherein the first and second steps areperformed under temperature of 200° C. or more and 300° C. or less. 13.The method according to claim 10, wherein the metal plate comprises: 8.3mass % or more and 9.5 mass % or less Al; 0.5 mass % or more and 1.5mass % or less Zn, and the remainder that comprises Mg and impurities.14. The method according to claim 10, further comprising the step of:heat treating the pressed product at a temperature of 100° C. to 450° C.